[Gasification] Fe + H2O = FeO + H2 -- Reply 3

[Peter Singfield]

At 05:14 PM 12/1/2006 -0500, you wrote:
>Dear List,
>
>Interesting information at below link:
>http://gasifiers.bioenergylists.org/?q=node/218
>
>
>Snip:
>"The above equation tells us that with steam and hot or molten iron we can
>make H2 with rust being the byproduct. This rust can be converted back to
>iron. Check out the below links for more information."
>
>
>Jeff

http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO2&Sect2=HITOFF&p=1&u=/net
ahtml/search-adv.htm&r=6&f=G&l=50&d=pall&S1=((H2+AND+Coal)+AND+Refining)&OS=
H2+AND+Coal+AND+Refining&RS=((H2+AND+Coal)+AND+Refining)

DETAILED DESCRIPTION OF THE INVENTION 

The invention provides process and apparatus for producing simultaneously a
high-purity, high-pressure hydrogen-rich gas stream and a high-purity,
high-pressure carbon monoxide-rich gas stream separately and continuously
using a molten metal gasifier that contains at least two zones, a "feed
zone" and an "oxidation zone", (or in a saving embodiment, a feed mode and
an oxidization mode) together with necessary ancillary equipment. Each zone
(mode) preferably operates at a pressure above 5 atmospheres absolute and
contains a bath of comprising molten iron and, possibly, other molten
metals, such as copper, zinc, chromium, manganese, nickel or other meltable
metal in which carbon is soluble. Preferably the bath contains at least 30
percent iron by weight. Depending upon the feed, the bath may also contain
slag components which, if present, preferably form a separate phase. 

In the feed zone, a hydrocarbon-containing feed in the form of a gas,
liquid, solid or mixed phase, e.g., a solid-liquid slurry or atomized solid
or liquid is introduced below the surface of the molten metal bath so that
the hydrocarbon feed comes into intimate contact with the molten metal. The
feed is introduced beneath the surface of the molten metal by a submerged
tuyere or lance or by high-velocity injection from a lance above the bath,
thereby ensuring that substantially complete chemical reactions and
substantially complete conversions to hydrogen and carbon are achieved. It
has been shown that high-purity hydrogen, defined as having a composition
very close to thermodynamic equilibrium, can be obtained in this manner.
The high-purity hydrogen thus formed leaves the feed zone as a
high-pressure hydrogen-rich gas, while the carbon dissolves in the molten
metal. Any nitrogen compounds present in the hydrocarbon feed will
decompose to form molecular nitrogen and leave as an impurity in the
hydrogen-rich gas. The hydrocarbon feed should contain a minimum of
moisture and other oxygen-containing compounds since these compounds will
decompose to form oxygen, which in turn will react with dissolved carbon to
form carbon monoxide, an undesirable impurity in the hydrogen-rich gas. 

The molten metal from the feed zone containing higher levels of dissolved
carbon then enters the oxidation zone where oxygen, air, oxygen-enriched
air or other suitable oxygen-bearing stream is introduced. The
oxygen-bearing stream is introduced beneath the surface of the molten metal
by a submerged tuyere or lance or by high-velocity injection from a lance
from above the bath. A portion of the dissolved carbon reacts with the
oxygen to form carbon monoxide. It has been shown that high-purity carbon
monoxide, defined as having a composition very close to thermodynamic
equilibrium, can be obtained in this manner. The high-purity carbon
monoxide thus formed leaves the oxidation zone as a high-pressure carbon
monoxide-rich gas separate from the hydrogen-rich gas produced in the feed
zone. The molten metal from the oxidation zone which has a lower
concentration of carbon re-enters the feed zone where the carbon level is
increased again. 

Both molten metal zones are operated at elevated pressures, preferably
between 5 and 100 atmospheres absolute, which results in the production of
the hydrogen-rich and carbon monoxide-rich gases at elevated pressures,
thereby eliminating the need for costly compression of the gases to
industrial operating pressures, as mentioned earlier. By reducing gas
hourly space velocity (GHSV), elevated pressures also result in smaller
equipment and piping for the process including all downstream equipment and
in reduced dust carryover from the feed and oxidation zones and, by Stoke's
Law, elevated pressures reduce deleterious dust carry-over or "fuming". 

A significant portion of the oxygen left in the molten iron as it re-enters
the feed zone will react with carbon from the hydrocarbon feed to form
carbon monoxide, which then becomes an impurity in the hydrogen-rich gas
stream. Thus, it is important to operate the process in such a manner that
there is a minimum of oxygen present in the molten iron when it re-enters
the feed zone. As a minimum, the molten metal will contain dissolved oxygen
based on the equilibrium with carbon monoxide gas. In addition, as an
oxygen-rich stream is introduced into molten metal, there is a tendency for
the oxygen solubility limit of the molten metal to be exceeded immediately
at the interface between the oxygen-rich stream and the molten metal, which
results in the formation of a separate iron oxide phase at the interface.
This iron oxide phase will be readily dissolved by surrounding molten metal
and not accumulate in the molten metal bath provided the overall oxygen
concentration of the molten metal bath is below the oxygen solubility
limit. If the overall equilibrium oxygen concentration of the molten metal
bath exceeds the solubility limit, however, the separate iron oxide phase
will tend to accumulate to significant levels. Then, when the molten metal
containing significant quantities of this iron oxide phase re-enters the
feed zone, much of this iron oxide phase will react with carbon from the
hydrocarbon feed to form a substantial quantity of carbon monoxide, which
will contaminate the hydrogen-rich gas being produced. Accumulation of
significant quantities of a separate iron oxide phase also substantially
increases the attack of the refractory walls in the vessels holding the
molten metal since a separate iron oxide phase can be very aggressive
toward refractory. Thus, the oxygen concentration in the molten metal must
be controlled so that it does not exceed its solubility limit. 

When molten iron is in equilibrium with carbon monoxide gas (formed in the
oxidation zone), it has been shown that carbon and oxygen exist in the
molten iron at equilibrium concentrations which can be determined by the
equation: ##EQU1## where: K is an equilibrium constant that varies with
temperature, dimensionless 

[C] is the concentration of carbon in molten iron, weight percent 

[O] is the concentration of oxygen in molten iron, weight percent 

P.sub.CO is the partial pressure of carbon monoxide, atmospheres absolute
(ata) 

T is the temperature, .degree. K. 

The solubility limit of oxygen in molten iron can be described by: 

log [O.sub.solubility limit ]=-6320/T+2.734 (2) 

where: 

[O.sub.solubility limit ] is the concentration of oxygen in molten iron at
its solubility limit, weight percent 

Thus, at a given temperature, T, the minimum concentration of carbon
required in the molten iron to ensure that the equilibrium oxygen
concentration in the molten iron does not exceed the oxygen solubility
limit can be described by the equation: ##EQU2## 

At 1600.degree. C., for example, the solubility limit of oxygen based on
Equation (2) is 0.229 weight percent in molten iron. Using Equation (3) at
this temperature, the minimum carbon concentrations as a function of
pressure required to prevent the equilibrium oxygen concentration from
exceeding 0.229 weight percent are calculated as follows: 


    ______________________________________
    CO Partial    Min. Carbon
    Pressure, ata Conc., wt %
    ______________________________________
    0.01          0.00009
    0.1           0.00088
    1             0.00884
    5             0.04422
    10            0.08844
    20            0.17688
    50            0.44221
    70            0.61910
    100           0.88443
    150           1.32665
    ______________________________________



Similar relationships can be determined for different temperatures and for
molten metal baths which contain iron mixed with other metals. 

In commercial steel-making practices, it is common to operate at a pressure
of one atmosphere and, in a few processes, under vacuum. As shown by data
above, when operating at carbon monoxide partial pressures of one
atmosphere or below, relatively low concentrations of carbon can be
achieved without reaching the oxygen solubility limit. For example, at
1600.degree. C. and one atmosphere, the carbon concentration must fall
below 0.0088 weight percent before the solubility limit of oxygen is
exceeded and a separate iron oxide phase starts to accumulate. 

When operating at elevated pressures, on the other hand, control of minimum
carbon levels becomes much more critical. At 1600.degree. C. and 100
atmospheres of pressure, for instance, the oxygen solubility limit is
reached when the carbon level reaches about 0.88 weight percent, which is
100 times higher than for one atmosphere of pressure. 

Thus, in the present invention, the carbon concentration in the molten iron
leaving the oxidation zone and entering the feed zone is controlled above
the value determined by Equation (3) at elevated pressures to prevent the
equilibrium oxygen level from exceeding its solubility limit and causing
the accumulation of a separate iron oxide phase, which would result in the
excessive formation of carbon monoxide in the feed zone and excessive
contamination of the hydrogen-rich gas. 

The carbon concentration in the molten metal bath leaving the feed zone, on
the other hand, is controlled at a higher concentration in order to
minimize the quantity and circulation rate of molten metal required in the
system. The economics of the process are better when the differential in
the carbon concentrations between the feed zone and the oxidation zone are
higher. Thus, the carbon concentration in the molten metal leaving the feed
zone should be maximized, although the concentration must be kept below the
carbon solubility limit (which is in the range of 4-5 weight percent in
molten iron) in order to minimize unreacted carbon and hydrocarbon feed
from leaving the molten metal as dust and lower molecular weight
hydrocarbons in the effluent gas. 

This invention also includes having the hydrogen-rich and carbon
monoxide-rich gases flow from the molten metal zones through separate
product gas lines and pass through successive downstream coolers and dust
removal systems to prepare the gases for use by industrial processes. 

Suitable feeds for the process include hydrogen- and carbon-containing
materials selected from the group consisting of: light gaseous hydrocarbons
such as methane, ethane, propane, butane, natural gas, and refinery gas;
heavier liquid hydrocarbons such as naphtha, kerosene, asphalt, hydrocarbon
residua produced by distillation or other treatment of crude oil, fuel oil,
cycle oil, slurry oil, gas oil, heavy crude oil, pitch, coal tars, coal
distillates, natural tar, crude bottoms, and used crankcase oil; solid
hydrogen-and carbon-containing materials, such as coke, coal, rubber, tar
sand, oil shale, and hydrocarbon polymers; and mixtures of the foregoing. 

A portion of the hydrogen-rich gas or carbon monoxide-rich gas may be
recycled in the process to facilitate feeding hydrocarbons to the feed zone
or feeding an oxygen source to the oxidation zone or to promote mixing or
movement of the molten metal. 

When feeding a heavier liquid or solid hydrocarbon to the feed zone and
feeding oxygen to the oxidation zone, the overall process of converting the
feedstock to hydrogen-rich and carbon monoxide-rich gases is exothermic.
Thus, it becomes necessary to moderate the temperatures of the process. In
the present invention, this is accomplished by (a) adding light gaseous
hydrocarbons to the feed zone, (b) adding carbon dioxide to the oxidation
zone, (c) adding steam to the oxidation zone or (d) diluting the oxygen
with air. In each case, sufficient material is added to achieve an overall
adiabatic operation and stable operating temperatures. Case (a) or (b) is
preferred when the objective is produce two high-purity gas products. Case
(c) or (d) introduces impurities to the carbon monoxide-rich gas and is
practical only if the purity of the carbon monoxide-rich gas is not critical. 

When a hydrocarbon feed containing sulfur compounds is fed to the feed
zone, the sulfur compounds will decompose and elemental sulfur thus formed
will dissolve in the molten metal. In conventional practice, a fluxing
agent, such as calcium oxide, is added to the bath to react with the
dissolved sulfur and produce a sulfide, which forms a slag phase which
tends to float on the top of the molten metal. The slag is removed
continuously or intermittently by tilting the vessel and pouring out the
slag or by allowing the slag to flow through a tap hole in the side of the
vessel. Pouring or tapping slag is difficult to practice in a vessel
operating at elevated pressures. To handle sulfur in hydrocarbon feeds
containing high levels of sulfur of up to 4 weight percent or more requires
the use of large amounts of fluxing agents and produces large amounts of
slag which must be disposed of safely. Thus, it is becomes very expensive
to handle hydrocarbon feeds containing high levels of sulfur using
conventional practices. 

As an added feature of the present invention, the sulfur in the hydrocarbon
feed is processed without the use of slag. Dissolved elemental sulfur (from
the hydrocarbon feed) is allowed to build up in the molten metal bath to an
equilibrium level and to react with hydrogen dissolved in the bath (also
from the hydrocarbon feed). Hydrogen sulfide is formed and leaves the
molten metal bath in the gaseous effluents, primarily the hydrogen-rich
gas. The concentration of elemental sulfur dissolved in the molten metal
bath will reach an equilibrium level such that the rate of sulfur leaving
the molten metal bath as hydrogen sulfide is equal to the rate of sulfur
entering the molten metal bath with the feed. The equilibrium concentration
of sulfur in the molten metal is a function of the carbon level present. By
achieving a relatively high level of carbon in the molten metal leaving the
feed zone, the equilibrium level of sulfur in the bath can be minimized.
Sulfur compounds other than hydrogen sulfide, such as carbonyl sulfide and
carbon disulfide, may also be formed and leave in the products gases,
especially in the carbon monoxide-rich gas. The product gases may be fed to
conventional scrubbers to remove the hydrogen sulfide and other gaseous
sulfur compounds, thereby recovering the sulfur for reuse in industry and
producing substantially sulfur-free product gases. 

As another added feature of the present invention, a portion of the liquid
hydrocarbon feed, prior to its introduction to the molten metal feed zone,
may be used as a scrubbing medium to remove dust from the hydrogen-rich and
carbon monoxide-rich gases (6524AUS). The portion of the hydrocarbon feed
containing the removed dust is then joined with the remainder of the
hydrocarbon feed and introduced to the feed zone, thereby providing a
direct and inexpensive means of recovering and recycling the dust back to
the molten metal bath. 

As still another added feature of the present invention, the liquid
hydrocarbon feed containing removed dust may be passed through a magnetic
separation device to preferentially separate out a portion of the low-iron
dust from the hydrocarbon feed before it is fed to the molten metal feed
zone. In this manner, a portion of the non-iron slag compounds which can
build up in the molten metal bath over time may be continuously removed
from the system. 

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